The present invention relates to a monitoring system for determining deviations of a beam of ionizing radiation from a predetermined axis, comprising a plurality of coaxially superposed ionization chambers of the kind described in my copending U.S. patent application Ser. No. 434,912 filed 21 Jan. 1974, now U.S. Pat. No. 3,942,012. The object of this invention is to provide an improved assembly of ion-collecting electrodes in these ionization chambers, yielding comparable results with a simpler structure and simpler processing circuits.